Synthetic resins are widely used as engineering plastics in a variety of end-uses, such as building materials and automobile parts. The engineering plastics have good physical and chemical resistance, and are low cost. A disadvantage of some engineering plastics is that they have poor impact strength. Poor impact strength of these materials may be overcome by blending impact modifiers with the resins.
Impact modifiers generally consist of low-Tg, elastomeric polymers. Unfortunately the low-Tg polymer particles are typically difficult to handle. They are tacky and tend to stick together (blockiness), forming clumps or agglomerates during processing and storage. The agglomerates may be difficult to separate and disperse into the engineering polymer matrix, leading to a less than optimal modification of the plastic.
Core shell impact modifiers typically have rigid high Tg polymers in their outmost have good anti-blocking properties and are easy to handle. They can also be spray-dried or coagulated.
It is known in the art that the rubber component of a core shell impact modifier provides the impact toughening properties. It is thus desired in the industry to maximize the rubber content in the modifier yet still maintain good anti-blocking properties and excellent powder handling characteristics.
Unfortunately, when the percentage of shell material is decreased, there is an increased likelihood for incomplete coverage of the particle. When the elastomeric core is incompletely covered, it can stick to other particles and form agglomerates. The rubbery agglomerates are difficult to process and also lead to poorer properties of the engineering plastics. Core-shell compositions exemplified in the art have shell levels of at least 25 percent and normally greater than 30 percent by weight to ensure complete coverage of the elastomeric core.
One method of preventing the formation of agglomerates is to blend hard particles with the impact modifier, as disclosed in U.S. Pat. Nos. 4,278,576, and 4,440,905. This method does not teach or demonstrate how to modify the core shell impact modifier itself so that improved shell coverage and better powder characteristics can be achieved.
Hydroxy alkyl (meth)acrylate monomers have been incorporated into the shell to improve compatiblization of the shell with the engineering plastic matrix. The use of hydroxy-functional monomers in the shell has been described in U.S. Pat. Nos. 5,321,056 and 5,409,967. These references describe particles having 10 to 60 percent shell, while teaching that the rubber phase concentration of the impact modifier composition be kept relatively low (column 4 lines 63–68), and exemplifying only particles having at least 35 percent shell.
JP 54-48850 describes the use of polymers made from hydroxyl-functional monomers for use as impact modifiers. In one instance, 10 to 40 percent of a hydroxyl-functional polymer latex is blended with 60 to 90 percent of a rubbery polymer latex and solidified with magnesium sulfate. The “rubbery polymer”, as described in Examples 1–3 is a core-shell polymer having a butadiene core and a styrene/acrylonitrile shell. The resulting blend consists of core/shell butadiene/styrene-acrylonitrile particles, and separate vinyl particles having hydroxyl groups. The core-shell polymer is never described as having hydroxy groups in the shell. In another embodiment a multilayer structure having 70 percent core and 30 percent hydroxy-functional shell is produced by sequential polymerization.
U.S. Pat. No. 6,130,290 describes a core-shell particle having a two-part shell. The outer shell contains a hydroxy alkyl (meth)acrylate copolymer, while the inner shell does not. The Examples describe particles having from 60–70 percent rubbery core and 30–40 percent of the multi-layered shell consisted of rigid high Tg polymers.
None of the prior arts teaches or demonstrates that the use of hydrophilic comonomers can affect the shell coverage on the rubber core and/or powder performance of core/shell type particles.
The problem solved by the present invention is to find a means to reduce the level of shell material without losing powder properties, in other words to provide more complete shell coverage when less than the usual amount of shell monomer is used.
While not being bound by any particular theory, it is believed that a significant portion of the shell monomer diffuses into the core, and polymerizes inside the core rather than on the surface of the core polymer. Thus, when the shell monomer mixture is polymerized, only some of the monomer mixture actually forms a polymer shell covering the core. Moreover, the polymer shell that forms may be of an uneven thickness and/or incomplete. Evidence of poor coverage can often be observed through the measurement of the minimum filming temperature of the core-shell polymer latex or through direct microscopy study (such as atomic force microscopy) of the core-shell polymer particles.
Surprisingly, it has been discovered that the use of low levels of hydrophilic monomers to form a copolymer in the shell layer leads to better shell coverage of the core polymer. Thermodynamic considerations suggest that a hydrophilic monomer, and a copolymer formed from the hydrophilic monomer should prefer to remain on the surface of the particle rather than migrating into the core. Shell coverage is improved by thermodynamic preference. Better efficiency of shell coverage means that less shell monomer needs to be used in order to attain good powder properties, allowing for larger elastomeric cores.